Thin film IGFETs are made in a 0.5-5 ohm-centimeter semiconductive layer, preferably a monocrystalline layer, about 0.1 to 0.5 micrometer thick. In such transistors the channel region occupies the entire thickness of the semiconductor layer. Thin film IGFETs have recently become of renewed interest because of their potential use as power switching transistors.
In a thin film depletion mode IGFET, source and drain contacts are disposed on the surface of a semiconductive thin film on opposite sides of a gate electrode. Areas of the film beneath the contacts form source and drain regions of the IGFET. These regions may be more highly doped, at least at their surface to reduce contact resistance. The film area between them, under the insulated gate electrode, forms the IGFET channel. With no gate potential applied, there is thus a moderate to low resistivity path between the source and drain contacts, depending on film resistivity. Applying a potential of appropriate polarity to the gate electrode depletes the channel of majority current carriers directly under the insulated gate electrode. With extremely thin films, i.e. less than 0.5 micrometer in thickness, the depletion can penetrate deeply enough with increased gate voltage to completely pinch off the channel. However, with films thick enough to be useful in power switching applications, i.e. 0.5 to 3 micrometer in thickness, pinch off could not heretofore be achieved. A report many years ago proposed that complete pinch off might be limited to the thinner films because of inversion layer formation at the semiconductive-gate insulator interface. Hence, complete current blocking in a power switching thin film depletion mode IGFET has not been obtainable.
I have now found how to eliminate, or at least significantly reduce, formation of the inversion layer. In substance, I preclude accumulation of minority current carriers at the channel surface in amounts sufficient to form an inversion layer. Since they do not significantly accumulate, they do not significantly counteract the channel field effect of the gate potential. In such instance channel thickness can be increased, which increases the power handling capability per unit area of the resultant channel and, of course, the IGFET of which it is a part. Hence, thin film depletion type IGFETs can now be used to more efficiently switch higher electrical currents.